1. The Technical Field
The present invention is directed to the field of fiber optics. More particularly, the present invention is directed to an apparatus and method for injecting light into an optical fiber.
2. The Prior Art
Commercial single mode optical fiber couplers typically are made by twisting two uncoated optical fibers together, heating the twisted region, and pulling on the fiber ends to reduce the diameter of the heated and softened region. When the fiber diameters are reduced sufficiently, the core modes overlap to produce the desired coupling. In principle, this method could be applied to couple active fibers without interrupting data transmission through the fibers, but the procedure would be very risky and would require complex, specialized equipment and precise control over the drawing process. The finished coupler would be permanent and non-adjustable.
Another method for coupling active fibers in a non-interrupting manner involves forming a macrobend, typically a constant-radius curve, in the subject fiber and injecting light through the bend into the core of the subject fiber. Such an injector can consist of a cleaved single mode fiber plus a suitable lens. Injection efficiency (the fraction of the light leaving the injector which actually couples into the core mode of the subject fiber) using the macrobend approach tends to be very low due to inherently poor coupling. In addition, the macrobend techniques are usually applied only to conventional polymer-coated fiber because of the reliability concerns associated with bare fiber. When injecting through the polymer coating, aligning the injection source with the single mode core of the bent fiber is extremely difficult, and the resulting coupling is very inefficient. The basic macrobend technique also can be used to extract light from an optical fiber. For example, some commercial xe2x80x9cfiber identifiers,xe2x80x9d used by fiber system installers, utilize the macrobend technique for extracting light to identify active fibers and light propagation direction.
The macrobend approach typically is not used for permanent couplers because of inherent inefficiencies, plus reliability concerns related to the fiber bending stresses. Bend-type couplers tend to subject the fiber to high bending stresses which are detrimental to the reliability of the fiber because of a phenomenon known as static fatigue. Static fatigue can cause a fiber in bending or tension to fail over time due to the propagation of cracks. The higher the initial stress a fiber is subjected to, the shorter its life is likely to be. Although it is known to use heat to anneal the bent fiber to remove the stress, this technique results in a permanent bend and, therefore, a coupler that is always xe2x80x9conxe2x80x9d and not adjustable. Also, elaborate equipment and precise control are required to practice this technique.
Accordingly, it would be desirable to provide an adjustable fiber optic coupler which allows efficient light injection into an active fiber to increase capacity without disrupting existing traffic through the fiber.
The present invention provides a method and apparatus for efficiently injecting light through the side and into the core of a single mode optical fiber as an alternative to the conventional approach of severing the fiber to splice in a pre-made coupler. The present invention makes possible, for example, the insertion of additional wavelengths into a fiber carrying WDM (wavelength division multiplexed) signals, without disrupting the existing traffic. Using the method and apparatus of the present invention, it is possible to upgrade the transmission capacity of an active fiber or network without shutting down the system, and without requiring an alternate, or xe2x80x9cprotect,xe2x80x9d fiber to temporarily carry the traffic.
A light injector according to the present invention is adjustable and may be switched on and off. It can handle high data rates as well as WDM signals. In addition, the reciprocal behavior of this bend-type device allows it to be used to extract, or xe2x80x9cdrop,xe2x80x9d an adjustable fraction of light from the fiber core into a detector or another fiber.
A light injector according to the present invention includes a traffic fiber, an injector or extractor fiber, a first fixture which secures the traffic fiber, and a second fixture which secures the injector or extractor fiber. The injector or extractor fiber preferably includes a lens for directing light to be injected therefrom to the traffic fiber or for collecting light extracted from the traffic fiber.
The fixtures can be combined in an integral unit or they can be separate units. In either case, the fixtures secure the fibers in a predetermined alignment with each other. The first fixture can have square edges where the traffic fiber exits it. Alternatively, the first fixture can include a curved or cylindrical mandrel about which the traffic fiber can be bent, as discussed further below. The mandrel can be movable in a direction parallel and/or perpendicular to the traffic fiber""s longitudinal axis, or centerline.
A depressor is provided to deflect a portion of the traffic fiber, thus creating a bend in the fiber. Preferably, the depressor is located within a few fiber diameters of the point where the traffic fiber exits the first fixture, namely, the transition point, although it can be located at greater distances from the transition point in some embodiments. The depressor can be integral with or separate from either or both of the two fixtures. The injector or extractor fiber can act as the depressor.